Calculate Volume for Triangular Prism..I think

OK....I'm trying to calculate the volume of my speaker boxes that I'm building for the heck of it. The box when standing it's tall height of 100", and you looking at it from the side is a right-angle triangle. Dimensions:H=100" W=12" L=20" My guess to the formula was a simple...l*w*h/2 And I get 196.68 L....Is this correct? With the regular l*w*h equation I get, 393.36L. Here's my thinking...Take the volume of the rectangular prism that is created by two speakers of right-angles and divide that volume by two. It seems right to me.:confused: